1. Field of the Invention
The present invention relates to a method of crosslinking organic polymers by reacting mutually reactive side groups, or by reacting a polymer with a low molecular weight crosslinking agent to form crosslinking bridges. "Crosslinking bridges" are understood to be short molecular chains which are disposed between the main chains of two polymer molecules and are covalently bonded with these compounds. Reactive "side groups" are understood to be side-disposed groups which participate in the formation of the crosslinking bridges, wherewith said side groups are not components of the main chain of the polymer and are not directly connected to said main chain.
2. Description of the Background
A known industrial method of crosslinking polymers is based on the reaction of formaldehyde with hydroxyl or amino groups, as in the preparation of aminoplast resins or in the preparation of acrylic polymers containing N-hydroxymethyl amide groups. Because of concern about possible adverse health effects of formaldehyde, crosslinking systems which do not employ formaldehyde have achieved in that search, there is still a demand for crosslinking methods which lead to crosslinking which is of the same value as that produced by the known formaldehyde systems, and which develop under similar conditions. The degree of crosslinking of acrylic polymers can be determined by swelling in tetrahydrofuran. The weight increase during the swelling and the weight loss attributable to dissolution of un-crosslinked parts of the polymer are parameters of this determination; both of these parameters decrease as crosslinking increases.
U.S. Pat. No. 3,200,099 discloses a method of crosslinking wherein hydroxyl-group-containing polymers are crosslinked with aldehydes or with acetals derived from aldehydes; the method involves formation of acetal bridges. The crosslinking agents primarily used are formaldehyde and/or formals derived from formaldehyde. However, acrolein, acrolein acetal, or acetone dimethylketal may be used instead. Klostermann, P., Guenther, P., and Stolzenbach, H. G., 1988, "Farbe und Lack" 94th annual edition, pp. 425-429, report on transacetalization as a principle of crosslinking in the formation of paint films. The acetal bridges which are produced are more acid-sensitive than crosslinked aminoplast resins or formaldehyde-crosslinked acrylamide copolymers.
Polyvinyl alcohol can be crosslinked with glutaraldehyde or epichlorohydrin, and can be converted to an ion exchanger with glyoxylic acid (Eur. 0S 24,055). The areas of applicability of crosslinked polyvinyl alcohol are narrowly limited; thus the technique does not offer a universally applicable method. Hydroxyl-group-containing polymers can also be crosslinked with diisocyanates or bisepoxides. However, these crosslinking agents have certain drawbacks. They tend to be toxic, to require drastic reaction conditions, or to deleteriously affect the chemical character of the polymers employed.
Self-crosslinking copolymers of acrylamidobutyraldehyde diethylacetal or similar polymerizable amidoacetals or -ketals are disclosed in Eur. 0S 201,693. They may also react with cellulose or polyvinyl alcohol, with formation of dioxolane- or dioxane structures. The crosslinking of the copolymers, with themselves or with diol groups of cellulose or polyvinyl alcohol, under the influence of acid catalysts, requires temperatures of about 150.degree. C.